1. Field of the Invention
This invention relates to energy curable resin compositions used to formulate energy curable printing inks. More particularly, the invention relates to energy curable printing inks containing acrylated rosin and methods for preparing the rosin.
2. Description of Related Art
Resins, derived from natural products such as rosin esters are widely used as printing-ink vehicles in flexographic and gravure printing inks. Maleic modified rosin-esters exhibit good pigment wetting, gloss retention, color retention and adhesion. Typically these esters are prepared from rosin, maleic anhydride and polyols, such as glycerol or pentaerythritol. In order to render these materials energy curable, acrylic groups are attached to free hydroxyl groups of the rosin ester. Since these hydroxyl groups are sterically hindered due to the bulky rosin moieties, the acrylation with acrylic acid is difficult, due to severe reaction conditions necessary. Often these severe reaction conditions cause undesired polymerization of the acrylic functions at high temperatures due to their thermal instability. Therefore, an introduction of acrylic groups under mild conditions is required.
WO 87/04448 describes the esterification of fumarated rosin esters with hydroxyethyl acrylate at high temperatures above 200° C. At such temperatures there is a high risk of polymerization of acrylated functions even in the presence of an inhibitor.
U.S. Pat. No. 4,035,320 describes the reaction of special rosin esters, prepared with a large excess of polyols, along with acrylic acid. According to the nature of the described special rosin esters the acrylated compositions exhibit a mixture of low levels of acrylated rosin-esters and free acrylated polyols. However, the high excess of remaining hydroxyl functions after the reaction can affect the properties of the ink, since greater number of hydroxyl often give poor lithographic performance properties. To compensate for this the patent discloses that the free hydroxyl groups are capped by an additional reaction with acetic anhydride.
There continues, however, to be a need for methods to efficiently produce acrylated rosin esters, wherein the acrylate groups are connected directly to the rosin ester, and are free of excess hydroxy groups and polymerized acrylate groups and wherein gellation, which can occur as a result of only partial polymerization during synthesis, is avoided.